The invention concerns, according to a first teaching, a sound damper for ultrasonic waves in a gas flow, with at least one damping element that can be inserted into a line for the gas flow.
Sound dampers for sound waves in the acoustic range (20 Hz-20 kHz) essentially have been known from the prior art for a long time. For example, they are inserted into motor vehicle exhaust gas flows to reduce emissions of sound waves in the acoustic range as much as possible, or even to eliminate them entirely.
In the attempt also to reduce emissions of sound waves in the acoustic range in industrial plants as much as possible, in order to meet environmental protection regulations, throttle valves to control a gas flow, which, instead of emitting sound waves caused by the throttling process in the environmentally-relevant acoustic range, now produce sound waves in the environmentally uncritical ultrasonic range ( greater than 20 kHz), especially between 2 kHz and 63 kHz, have recently become known.
Such emission of ultrasonic waves by modem throttle valves, on the one hand, meet the environmental standards, but on the other hand, result in conflicts with flow meters frequently used in control circuits to adjust a gas flow, which operate according to the ultrasonic principle. The ultrasonic waves produced by the modem throttle valves in the known flow meters, which operate on the ultrasonic principle, lead to very considerable malfunctions, up to total failure.
It is therefore a first object of the invention to provide sound dampers in the ultrasonic range, especially for flow meters that work on the ultrasonic principle.
According to the first teaching of the invention, the object set forth and explained previously is solved by locating at least one scattering unit having a plurality of small reflective sudden in the damping element.
The configuration in accordance with the invention guarantees that the ultrasonic waves are reflected very frequently, so that there is a great probability of destructive interferencexe2x80x94and hence cancellationxe2x80x94of the ultrasonic waves on the extended acoustic paths, which leads to a good damping of the amplitude of the ultrasonic waves.
If the dimensions of the reflective surfaces vary approximately within the range of the wavelength of the ultrasonic waves, this also guarantees that the largest possible number of reflective surfaces effective for the ultrasonic waves in a given volume can be provided. Reflective sure with dimensions in the range below the wavelength of the ultrasonic waves are not xe2x80x9cvisiblexe2x80x9d or only conditionally so, while reflective surfaces in the range above the wavelength of the ultrasonic waves are xe2x80x9cvisiblexe2x80x9d for the ultrasonic waves and thus result in reflection, but at the same time take up a lot of space unnecessarily.
The probability of destructive interference connected with the longest possible acoustic paths is particularly high if the reflective faces are oriented irregularly.
A further improvement in the damping of the sound damper for ultrasonic waves in accordance with the invention is guaranteed by the fact that the reflective surfaces of the scattering units are at least partially curved. Such curving of the reflective surfaces leads to the formation of eddies within the gas flow, which also result in a damping of the amplitude of the ultrasonic waves.
If a layer of gauze or a comparable textile material is located on and/or in the damping element, a further reduction in the amplitude of the ultrasonic waves is guaranteed by the plurality of disturbances in the flow of gas through the gauze.
The sound damper for ultrasonic waves in accordance with the invention receives a particularly advantageous refinement when the damping element, comparable with known conventional damping elements for sound waves in the acoustic range, is made of a perforated, tubular body closed at one end. This type of damping element is simple and inexpensive to produce.
Experimentally, a diameter of approximately 20 mm for the holes in the damping element has proven especially advantageous for the damping properties of the sound damper.
In the damping element described, consisting of a perforated, tubular body closed at one end, a scattering unit having the arrangement of at least one open geometric structure pointing from outside to inside on the inside of the damping element has proven particularly advantageous. The open geometric structures can have completely different cross sections. For example, they can be made circular, square, or the like.
The sound damper in accordance with the invention receives a further advantageous refinement when the damping element is filled with a number of scattering units. This measure guarantees that the reflective surfaces are oriented highly irregularly, and the scattering units at the same time do not have to have an exessive complicated structure.
If the damping element is filled with a number of scattering units, so-called pall rings are particularly advantageous as the scattering units. On the one hand, these pall rings have a number of small reflective surfaces, and, on the other hand, these reflective surfaces are arranged in such a way that there is a high probability that eddies causing a further damping of the amplitude of the ultrasonic waves will be generated.
Comparable with sound dampers for sound waves in the acoustic range, it is also particularly advantageous for the sound damper in accordance with the invention to design the damping element adapted to a curved line. The acoustic paths, further elongated by this curvature, lead to an additional damping of the amplitude of the ultrasonic waves.
According to a second teaching, the invention concerns a flow meter which operates on the ultrasonic principle. Such a flow meter is known, for example from D-A-195 30 807.
According to the second teaching of the invention, a known flow meter which operates on the ultrasonic principle is characterized by the fact that a sound damper for ultrasonic waves, especially as described above, is located between an ultrasound source and the flow meter. This measure advantageously guarantees that the function of the flow meter, which works on the ultrasonic principle, is not influenced by ultrasonic waves from outside sources.